692 



ECOLOGY AND EVOLUTION 



organism. A part of an animal, for example 

 the tail of a lizard, may be shed when the 

 animal is attacked, an action that has been 

 thought to contribute to the survival of the 

 individual, which then regenerates a new 

 tail. Brilliant coloration of the tail may 

 even be associated with the specific 

 mechanisms for shedding the tail without 

 loss of blood. 



Maternal instinct subjects the individual 

 mother to a higher mortaUty rate when she 

 attempts to protect her young or eggs 

 (Haldane, 1932, p. 207; see also 339, 

 416). A honeybee worker is often killed 

 while protecting the colony. The sting of 

 the worker is barbed and is often left in 

 the stung animal together with a portion of 

 the abdomen. In contrast, the queen has a 

 smooth unbarbed sting. A soldier termite 

 has highly evolved defensive adaptations, 

 frequently is killed defending the society, 

 and exhibits regressive evolution of the re- 

 productive organs. During periods of 

 disturbance, the soldiers of some species 

 (Nasutitermes, Coptotermes, and the like) 

 concentrate at the point of danger (Fig. 

 149). In none of these does the individual 

 in question typically differ genetically from 

 other individuals making up the unit popu- 

 lation. Physiological and environmental 

 thresholds determine the manifestation of 

 the function of the individual, but the func- 

 tion itself may well determine the survival 

 of the reproductive individual, thus per- 

 petuating any genetic enhancement of func- 

 tional efficiency. 



Wright (1945) develops a mathematical 

 genetic theory to explain the possibility of 

 the fixation of a character valuable to the 

 population, but disadvantageous at a 

 given time to the individual organisms. He 

 believes that some form of intergroup selec- 

 tion is necessary for the establishment of 

 socially advantageous but individually dis- 

 advantageous mutations. Conditions for 

 such creative evolution of social units seem 

 to be met among the social insects. Con- 

 siderable inbreeding within small, partially 

 isolated populations occurs, together with 

 occasional crossing between reproductives 

 from different colonies. Intraspecies but in- 

 tercolony competition may occur. 



If whole populations are adaptive, it 

 seems possible that adaptations producing 

 beneficial death of the individual-death 

 for the benefit of the population— might 



evolve.* Numerous cases of beneficial 

 death are known at the level of the multi- 

 cellular organism. For instance, the sap- 

 wood cells of a tree function after their 

 death when they then transport water and 

 minerals from the roots to the leaves. The 

 functional insect wing is largely a dead 

 structure (only some living cells, glands, 

 and blood may be present), and in the 

 termite wing, a basal suture has evolved 

 that enables the outer portion of the wing 

 to be efficiently discarded after the coloniz- 

 ing flight. At the sexual level, the male is 

 often eaten by the female after copulation 

 (p. 690). At the population level, the 

 tumble weed or Russian thistle {Salsola 

 pestifer), introduced into the dry semides- 

 erts and prairies of North America from 

 the steppes of Russia, breaks off near the 

 ground, and its dried dead branches form 

 a stiff round mass that, when rolled over 

 the plains by the wind, disperses its seeds. 

 This obviously does not benefit the individ- 

 ual plant, but insures the spread and ulti- 

 mate survival of the new generation. From 

 these considerations, it follows that the 

 physiological effects of aging and senes- 

 cence may be adaptations for the benefit 

 of the species (see Physiological Longevity, 

 p. 273). 



Cannibahsm, as exhibited by numerous 

 groups of insects (p. 370), may benefit 

 the population system. This behavior pat- 

 tern causing the beneficial death of individ- 

 uals in overcrowded populations may be 

 the result of adaptation within a popula- 

 tion system. 



Pearl (1930a) states: "No death of an 

 individual occurring in . . . the post-re- 

 productive period can possibly be selective, 

 in the sense of having any effect upon the 

 race." Pearl here seems to view the repro- 

 ductive individual as the only unit upon 

 which selection may act. We do not at all 

 agree to this limitation. If the postrepro- 

 ductive (or for that matter, sterile) individ- 

 ual is integrated wdthin a supraorganismic 

 unit, its selection may influence survival of 

 the reproductive individuals and thus in- 

 directly have a profound influence upon 

 the further evolution of the race, 



" Beneficial death is defined here in a specific 

 way as it is related to that mortality resulting 

 from particular adaptations. In other places 

 (pp. 418, 603, 685, 706) we stress the fact 

 that death may be beneficial without such 

 adaptational implication. 



